Data communication typically occurs as the transfer of information from one communication device to another. This is typically accomplished by the use of a modem located at each communication endpoint. In the past, the term modem denoted a piece of communication apparatus that performed a modulation and demodulation function, hence the term “modem”. Today, the term modem is typically used to denote any piece of communication apparatus that enables the transfer of data and voice information from one location to another. For example, modem communication systems use many different technologies to perform the transfer of information from one location to another. Digital subscriber line (DSL) technology is one vehicle for such transfer of information. DSL technology uses the widely available subscriber loop, the copper wire pair that extends from a telephone company central office to a residential location, over which communication services, including the exchange of voice and data, may be provisioned. DSL devices can be referred to as modems, or, more accurately, transceivers, which connect the telephone company central office to the user, or remote location typically, referred to as the customer premises. DSL communication devices utilize different types of modulation schemes and achieve widely varying communication rates. However, even the slowest DSL communications devices achieve data rates far in excess of conventional point-to-point modems.
DSL transceivers can be used to provision a variety of communication services using, for example, asynchronous transfer mode (ATM). ATM defines a communication protocol in which 53 octet (byte) cells are used to carry information over the DSL communication channel. The first five octets of the ATM cell are typically used for overhead and the remaining 48 octets are used to carry payload data. When using a switched-carrier transmission methodology, a control transceiver may be connected via the DSL to one or more remote transceivers. In such a communication scheme, the transmission is commonly referred to as “half-duplex,” which is defined as two way electronic communication that takes place in only one direction at a time. With only a single remote transceiver on a line, switched-carrier transmission may instead be employed in full-duplex mode (i.e., allowing transmission in both directions simultaneously). In this case, full-duplex operation is typically enabled by employing either echo cancellation or frequency division multiplexing. Hybrid techniques are possible such as one in which there are multiple remote transceivers and communication takes place between the control transceiver and only one remote transceiver in full-duplex fashion. As it relates to the present invention, the common characteristic of these communication techniques is the use of a switched-carrier modulation in which transmitters are deliberately silent for some interval between signal transmissions. For simplicity, the following discussions assume the simplest case of using switch carrier modulation with a half-duplex (also sometimes referred to as “time domain duplex”) line usage discipline.
Before the transmission of ATM cells, a preamble containing channel, transmission, address and administrative information may be transmitted by the transceiver. The application of this preamble is sometimes referred to as “framing” the data to be transmitted. Due to the switched-carrier nature of the transmission, silence precedes this preamble and it is of course important for all symbols in this preamble to be received error free. It is also desirable to have the ability to precisely delimit the beginning and end of a transmission to within one transmitted symbol interval. Robustly delimiting the beginning of a message enables a receiving transceiver to reliably begin immediately decoding the message at the correct symbol. Likewise, robustly delimiting the end of a message enables a receiving transceiver to reliably decode the entire message through the final symbol and then stopping so as to prevent data loss and to prevent the inclusion of any false data. Furthermore, by communicating the end of message indicator to a receiving transceiver prior to the actual end of the message, line turnaround time (i.e., idle time on the line between transmissions) can be reduced, thereby increasing the effective use of the available line bandwidth.
Because the most efficient signal constellation encoding cannot allocate signal space to silence, it is impractical to reliably discriminate silence from a signal when analyzing only a single symbol encoding an arbitrary data value.
To improve message delimiting, existing techniques use special marker symbols whose symbol indices are greater than those used to encode data. At N bits per symbol (bps) data is encoded using symbol indices 0 through 2N−1. The special symbols use indices 2N and above. While these special marker symbols are useful for marking the beginning and end of a transmission, their placement at the outer edges of a constellation raises the peak signal, thus increasing the peak to average ratio (PAR) across all data rates by as much as 4 dB. Unfortunately, discrimination of special symbols has the same error threshold as does decoding of data.
Thus, it would be desirable to have a robust manner in which to detect the beginning and end of a transmission so that line bandwidth can be most efficiently allocated. Furthermore, it would be desirable to robustly transmit a message preamble including control information thereby greatly improving the probability that the preamble is received error free.